1. Field of the Invention
The present invention relates to a method of reducing resistance for a conductive film formed on a base material, more particularly it relates to a method of reducing resistance for a conductive film formed on a plastic base material.
2. Description of the Related Art
The conductive film such as ITO is increasing its demand in various application fields such as transparent electrodes in liquid crystal display elements, plasma-emission display elements, solar cells, or optical devices.
In order to build up a conductive film on a base material, usually amorphous metal oxide or crystalline metal oxide is formed on a base material heating the base material at above 200xc2x0 C. by means of electron beam deposition method (EB method), sputtering method, or ion-plating method. When the heating process of the base material is omitted, the film formation is generally feasible only for amorphous metal oxides that usually exhibit high resistance figures, yielding films of high resistance. Heating a base plate above 200xc2x0 C. accelerates crystallization of the oxide yielding polycrystalline metal oxide film. In general, polycrystalline metal oxides yield films of lower resistance than those of amorphous oxides.
Adoption of the method for formation of a polycrystalline conductive film having low resistance on a plastic base material is difficult in view of the heat resistant property of present plastic base materials. Thus it is not still implemented except for a few base plates made from highly heat resistant plastic base plates.
ITO is widely applied for an electro-conductive film of excellent transparency. Its light absorption, however, is not negligible depending on its use. Generally the light absorption is influenced greatly by the film thickness: it increases as the thickness increases. On the other hand, a thinner film brings about a larger electric resistance value although the light absorption decreases. Therefore, a new electro-conductive film is hoped for, which realizes negligible light absorption together with very small electric resistance. Such ITO films are not yet materialized at present. The requirement is still difficult to meet since the heat resistance of an ordinary plastic base material is low (223xc2x0 C. at most) at preparing an electro-conductive film on a surface of a plastic base material.
In contrast, a new electro-conductive film replacing ITO is described in Japanese Published Unexamined Patent Application No. Hei 11-302017, which is characterized by a complex metal oxide containing indium (In), antimony (Sb), and oxygen (O). This boasts low electric resistance, high transparency in the visible light region, and less content of expensive In. The complex metal oxide has a defected fluorite structure having a general formula In3Sb1-XO7-xcex4 (where xe2x88x920.2xe2x89xa6Xxe2x89xa60.2 and xe2x88x920.5xe2x89xa6xcex4xe2x89xa60.5), and the transparent film is manufactured by sputtering the above complex metal oxide as a target material. This method is characterized by execution of sputtering on a base plate at a high temperature of 500xc2x0 C., which yields a crystalline film. Naturally this cannot be applied to the formation of electro-conductive film on a plastic base material.
The above patent publication also describes a process of producing oxygen vacancies by reductive annealing of the obtained crystalline film for 0.1 to 10 hours at a temperature between 100xc2x0 C. and 1,300xc2x0 C., which enables injection of carrier electrons evolved by the charge compensation into the vacancies. Note that the annealing means annealing a crystalline film, not converting an amorphous film into a crystalline film.
With regard to the production of crystalline film by applying heat on an amorphous film formed on the surface of a plastic resin base, methods of forming a semiconductor film on a plastic resin base have been reported. For example, Japanese Published Unexamined Patent Application No. Hei 06-11738 describes a method of producing crystalline semiconductor film of silicon for an MIM apparatus. In the patent, a film surface made of insulating silicone base-compound is irradiated with an energy beam like laser beam inducing meltdown of the surface and converting the surface layer into a crystalline silicon film, still leaving the under layer as the insulating silicon base-layer. Also Japanese Published Unexamined Patent Application No. Hei 05-315361 describes the formation of a semiconductor film on a plastic film, in which a layer of non-crystalline material and a layer of insulating metal oxide are formed on a plastic film in this order, followed by application of laser beam on the side of an insulating oxide layer. The result is meltdown of the layer of non-crystalline material partly at a section close to an interface between the layer of non-crystalline material and the layer of insulating metal oxide. This does not give thermal damage by the laser beam on the plastic film and allows successful production of crystalline semiconductor film. Further, Japanese Published Unexamined Patent Application No. Hei 05-326402 reports a similar production method of multi-crystalline silicon layer on a plastic film. In the patent, a heat barrier layer is first formed on a plastic film in order to cancel the thermal effect of laser beam. Then a layer of amorphous silicon is formed on the barrier layer, followed by application of laser beam for converting the layer into amorphous silicon.
All these methods depend on the annealing process of an amorphous semiconductor film by laser beam for crystallization. They bring about meltdown of only the surface of amorphous semiconductor layer for avoiding thermal effects by laser beam (the temperature sometimes reaches 1,000xc2x0 C.) for crystallization, or in creating a heat barrier layer. Therefore, these methods are incapable of crystallizing a whole amorphous film, and require other processes such as creating a heat barrier layer as well as an expensive laser beam irradiation apparatus. In addition, it becomes necessary to scan the focus of laser beam all over the film surface, leading to difficulties in large size processing of the film. There is also a disadvantage of needing a long time for its complete crystallization.
The present invention has been contrived in view of the abovecircumstances. The present invention provides a method of reducing resistance of a conductive film at low temperatures depending on simple processes, more particularly a method of reducing resistance of a conductive film formed on a plastic base material. The method may be a method of reducing resistance for a conductive film made of metal oxide formed on a base material, which includes a UV light irradiation process on the film in vacuum or in an atmosphere of reducing gas at a temperature maintained between 25xc2x0 C. and 300xc2x0 C.
The conductive film made of metal oxide may be a conductive film including amorphous metal oxide.
The atmosphere of reducing gas may be an atmosphere of reducing gas containing hydrogen.
The metal oxide may be indium oxide or oxides of other metals containing indium oxide.
The metal oxide may be tin oxide or oxide of other metals containing tin oxide.
The metal oxide may be oxide of other metals containing both indium oxide and tin oxide.
The metal oxide may be zinc oxide or oxide of other metals containing zinc oxide.
The UV light may be emitted from an excimer lamp.
The maintained ranges of the temperature may be between 50xc2x0 C. and 250xc2x0 C.
The base material may be made up of a plastic base material and the abovementioned maintained ranges between 50xc2x0 C. and a heat resistant temperature of the plastic base material.
The heat resistant temperature of the abovementioned plastic base material may ranges between 100xc2x0 C. and 230xc2x0 C.
The conductive film including amorphous metal oxide may be formed by a sputtering method.
The conductive film including amorphous metal oxide may be formed by an electron beam deposition method.
The conductive film including amorphous metal oxide may be formed by an ion plating method.
A laminated material may be provided according to the present invention, which comprising a plastic base and crystalline metal oxide containing indium oxide and tin oxide that is formed and treated for reducing resistance on the plastic base. The oxide surface of the laminated sheet may have a sheet resistance ranges between 50 xcexa9/xe2x96xa1 and 10-xcexa9/xe2x96xa1.
The crystalline metal oxide containing indium oxide and tin oxide may exhibit definite peaks in X-ray crystallography.
A substrate for a device may also be provided according to the present invention, which including a laminated material that contains a plastic base and crystalline metal oxide containing indium oxide and tin oxide that is formed and treated for reducing resistance on the plastic base.